1. Field of the Invention
The invention relates to a thermal head which thermally transfers color material of an ink ribbon onto a printing medium, and a printer including the thermal head.
2. Description of the Related Art
As a printer for printing images and characters on a printing medium, such a thermal transfer type printer (hereinafter referred to as printer) is known which sublimates color material of an ink layer formed on one surface of an ink ribbon and thermally transfers the color material onto a printing medium to print color images and characters thereon. This type of printer includes a thermal head for thermally transferring the color material of the ink ribbon onto the printing medium, and a platen disposed at a position opposed to the thermal head to support the ink ribbon and the printing medium.
In this printer, the ink ribbon disposed on the thermal head side and the printing medium on the platen side overlap with each other. The ink ribbon and the printing medium move between the thermal head and the platen while being pressed onto the thermal head by the platen. During this period, the printer applies thermal energy to the ink layer from the back surface of the ink ribbon by using the thermal head and sublimates the color material through utilization of the thermal energy, thereby thermally transferring the color material onto the printing medium and printing color images and characters thereon.
According to this thermal transfer type printer, the power consumption of the printer is large since prompt increase of the temperature of the thermal head by heating is necessary at the time of high-speed printing. It is therefore difficult, particularly for a household printer, to increase the printing speed while saving power. For achieving high-speed printing by the household thermal transfer type printer, it is necessary to increase thermal efficiency of the thermal head while decreasing power consumption.
A thermal head 100 shown in FIG. 20 is an example of a thermal head included in a thermal transfer type printer in related art. The thermal head 100 has a glass layer 102 on a ceramic substrate 101, and a heating resistor 103, a pair of electrodes 104a and 104b for causing the heating resistor 103 to generate heat, and a protection layer 105 for protecting the heating resistor 103 and the electrodes 104a and 104b in this order. According to the structure of the thermal head 100, an area exposed between the pair of the electrodes 104a and 104b becomes a heating area 103a which generates heat. The glass layer 102 is substantially circular-arc-shaped so that the heating area 103a can be opposed to an ink ribbon and a printing medium.
Since the thermal head 100 uses the ceramic substrate 101 having high thermal conductivity, thermal energy generated from the heating area 103a is released from the glass layer 102 through the ceramic substrate 101. Thus, the temperature immediately drops with excellent responsiveness. However, because the temperature of the thermal head 100 easily lowers due to the structure in which the thermal energy from the heating area 103a is released toward the ceramic substrate 101, the power consumption necessary for raising the temperature to the sublimation temperature increases and thus thermal efficiency decreases. According to the thermal head 100 which has high responsiveness but low thermal efficiency, it is necessary to heat the heating area 103a for a long time so as to obtain a desired concentration. As a result, the power consumption rises, and therefore increase in printing speed with power saving is difficult to achieve.
In order to overcome these drawbacks, the present inventors developed a thermal head 110 shown in FIG. 21. This thermal head 110 is now explained as art related to the invention. The thermal head 110 uses not a ceramic substrate but a glass layer 111 having lower thermal conductivity than that of the ceramic substrate so as to prevent transmission of thermal energy toward the substrate at the time of thermal transfer of color material onto a printing medium. According to the structure of the thermal head 110, a heating resistor 112, a pair of electrodes 113a and 113b, and a protection layer 114 are formed in this order on the glass layer 111 which has a substantially circular-arc-shaped projecting portion 111a. The projecting portion 111a of the glass layer 111 is exposed between the pair of the electrodes 113a and 113b, and has a substantially circular-arc shape so that a heating area 112a of the heating resistor 112 can be opposed to the ink ribbon and the printing medium.
Since the glass layer 111 having lower thermal conductivity than that of the ceramic substrate 101 shown in FIG. 20 functions as the ceramic substrate 101 in the thermal head 110, thermal energy generated from the heating area 112a is not easily released toward the glass layer 111. As a result, the quantity of heat supplied to the ink ribbon increases in the thermal head 110, and the temperature immediately rises at the time of thermal transfer of the color material onto the printing medium. Thus, the power consumption necessary for raising the temperature to the sublimation temperature of the color material decreases, which leads to improvement of thermal efficiency. However, since the thermal energy accumulated on the glass layer 111 is not easily released in the thermal head 110, the temperature does not immediately drop due to the presence of the thermal energy accumulated on the glass layer 111. Thus, the responsiveness lowers in contrast to the thermal head 100, and the printing speed of the thermal head 110 having low responsiveness is difficult to increase though its thermal efficiency is improved.
For achieving high-speed printing of high-quality images and characters with reduced power consumption, it is desirable that a thermal transfer type printer has both high thermal efficiency which is insufficient in the case of the thermal head 100 and high responsiveness which is insufficient in the case of the thermal head 110. Thus, the present inventors further developed a thermal head 120 shown in FIG. 22. This thermal head 120 is now discussed as other art related to the invention. Similarly to the thermal head 110 described above, the thermal head 120 includes a glass layer 121 having a substantially circular-arc-shaped projecting portion 121a, and a heating resistor 122, a pair of electrodes 123a and 123b, and a protection layer 124 are formed on the glass layer 121 in this order. The projecting portion 121a is formed such that a heating area 122a of the heating resistor 122 exposed between the pair of the electrodes 123a and 123b can be opposed to an ink ribbon and a printing medium. A groove 125 filled with air is formed inside the glass layer 121.
According to the thermal head 120 having the groove 125 on the glass layer 121, thermal conductivity of the groove 125 decreases due to the characteristic of the air having lower thermal conductivity than that of glass. As a result, heat release toward the glass layer 121 is further reduced compared with the thermal head 100 using the ceramic substrate 101 shown in FIG. 20. In this case, the quantity of heat supplied to the ink ribbon increases in the thermal head 120, and therefore the power consumption necessary for raising the temperature to the sublimation temperature of color material decreases and thermal efficiency increases. Moreover, since the thickness of the glass layer 121 is reduced by providing the groove 125 on the glass layer 121 in the thermal head 120, the quantity of accumulated heat on the glass layer 121 decreases and thus the thermal energy accumulated in the glass layer 121 can be released in a shorter time than in the case of the thermal head 110 having no groove on the glass layer 111 shown in FIG. 21. As a result, the temperature rapidly drops when the color material is not thermally transferred, which contributes to higher responsiveness. Accordingly, the thermal head 120 improves both thermal efficiency and responsiveness by providing the groove 125 on the glass layer 121. That is, the thermal head 120 can solve both the drawback of the thermal head 100 and the drawback of the thermal head 110.
As illustrated in FIG. 23, the thermal head 120 is affixed to a heat release member 126 for releasing thermal energy generated from the heating area 122a by adhesive in most cases. In addition, a semiconductor chip 127 having a driving circuit for driving the heating resistor 122 is provided on the same surface of the glass layer 121 as the surface where the heating resistor 122, the pair of the electrodes 123a and 123b, and the protection layer 124 are provided, and the semiconductor chip 127 is electrically connected with the electrode 123b by a wire 128 in most cases.
There is a demand for a miniaturization of a printer using the thermal head 120, particularly in the case of a household printer. In order to reduce the size of the printer, miniaturization of the thermal head 120 is necessary.
However, since the semiconductor chip 127 is disposed on the same surface of the glass layer 121 as the surface where the heating resistor 122 and other components are located in the thermal head 120, the size of the glass layer 121 is inevitably large. Therefore, miniaturization of the thermal head 120 and thus size reduction of the printer are difficult. Additionally, the cost increases since the large-sized glass layer 121 is used in the thermal head 120.
As illustrated in FIG. 23, the thermal head 120 is affixed to the heat release member 126 for releasing thermal energy from the heating area 122a by adhesive, and the semiconductor chip 127 having the driving circuit for driving the heating area 122a is provided on the same surface of the glass layer 121 as the surface where the heating resistor 122, the pair of the electrodes 123a and 123b, and the protection layer 124. The semiconductor chip 127 is electrically connected with the electrode 123b facing to the semiconductor chip 127 by the wire 128. The semiconductor chip 127 is higher than a portion where the heating area 122a is provided in the thermal head 120. Thus, in the printer using the thermal head 120, it is necessary to dispose the positions of moving paths of an ink ribbon and a printing medium away from the thermal head 120 so that the ink ribbon and the printing medium do not contact the semiconductor chip 127. This requirement imposes limitation on the locations of the moving paths of the ink ribbon and the printing medium.
There is a demand for miniaturization of a printer using the thermal head 120, particularly in the case of a household printer. In order to miniaturize the printer, size reduction of the thermal head 120 is necessary.
In the case of the thermal head 120, the ink ribbon and the printing medium moving between the thermal head 120 and the platen are positioned substantially perpendicular to the thermal head 120 so that color material can be appropriately transferred onto the printing medium by heat during movement of the ink ribbon and the printing medium between the thermal head 120 and the platen. When the movement of the ink ribbon and the printing medium is substantially perpendicular to the thermal head 120 in the printer, there is a possibility of contact between the semiconductor chip 127 and the ink ribbon and the printing medium since the semiconductor chip 127 is higher than the portion having the heating area 122a. In the structure of the thermal head 120, therefore, it is necessary to dispose the semiconductor chip 127 away from the portion of the heating area 122a so that the contact between the semiconductor chip 127 and the ink ribbon and the printing medium can be avoided. This requirement increases the size of the glass layer 121 of the thermal head 120, and therefore the cost rises and miniaturization becomes difficult.
In order to overcome these drawbacks, the present inventors further developed a thermal head 130 shown in FIG. 24. The thermal head 130 is now discussed as further art related to the invention. Similarly to the thermal head 120 described above, the thermal head 130 includes a glass layer 131 having a substantially circular-arc-shaped projecting portion 131a, and a heating resistor 132, a pair of electrodes 133a and 133b, and a protection layer 134 are formed on the glass layer 131 in this order. The projecting portion 131a is formed such that a heating area 132a of the heating resistor 132 exposed between the pair of the electrodes 133a and 133b can be opposed to an ink ribbon and a printing medium. A groove 135 filled with air is formed inside the glass layer 131. The thermal head 130 is affixed to a heat release member 136 by adhesive. According to the thermal head 130, a semiconductor chip 136 is not provided on the glass layer 131 but on another component as a rigid substrate 137. In the thermal head 130, the electrode 133b facing to the semiconductor chip 136 is electrically connected with a connection terminal 138 of the semiconductor chip 136 provided on the rigid substrate 137 by a wire 139, and the wire bonding portion is sealed by resin 140. According to the thermal head 130, the size of the glass layer 131 is reduced compared with the case of the thermal head 120, and therefore the cost is lowered.
According to the structure of the thermal head 130, the height of the semiconductor chip 136 is smaller than the height of the portion having the heating area 132a. However, there is a possibility that the wire bonding portion between the electrode 133b on the glass layer 131 and the connection terminal 138 on the rigid substrate 137 is positioned higher than the portion of the heating area 132a. Thus, even in the thermal head 130, the positions of the moving paths of the ink ribbon and the printing medium are limited with a necessity for disposing the wire bonding portion away from the portion of the heating area 132a. This requirement makes miniaturization difficult. Accordingly, even in the case of the printer using the thermal head 130, the positions of the moving paths of the ink ribbon and the printing medium moving in the vicinity of the thermal head 130 are limited.
JP-A-8-216443 is an example of related art.